TAMPA - Massive slabs of concrete from an elevated highway under construction in the median of the Lee Roy Selmon Crosstown Expressway collapsed Tuesday morning, narrowly missing rush-hour commuters passing by on either side.

Thousands of pounds of concrete folded like a piece of paper into a V-shaped crease. Fist-sized concrete chunks rained onto the road below, striking four vehicles. Two construction workers were hospitalized with minor injuries.

Fortunately, the bulk of the huge structure collapsed in on itself.

"This was the best-case senario," said Trooper Larry Coggins, spokesman for the Florida Highway Patrol, as early morning commuters were steered away from the damaged sites. "A catastrophe was diverted. It just went straight down."

The elevated bridge being built near 50th Street began rumbling under dark skies as suburban commuters clogged the connector to downtown Tampa. At 7:15 a.m., nearby toll booth workers heard a clap that sounded like thunder.

"Please tell me that's not thunder," toll collector Alma Marshall said to a coworker. "We looked at the weather and said it's going to storm. And lo and behold ..."

Authorities blamed a deep sinkhole that swallowed about 15 feet of a support column. The road then buckled and fell onto heavy construction equipment that authorities believe helped push open the undetected sinkhole.

Investigators will review a private company's soil tests, which did not detect sinkholes. The Tampa-Hillsborough Expressway Authority declined Tuesday to release the reports of those tests. Bridge builder PCL Civil Constructors also is facing state scrutiny after cracks were discovered in the contractor's work on the problem-plagued Memorial Causeway Bridge in Clearwater.

The authority does not know when repairs to the $350-million project will be finished or how much it will cost. They expect to take at least a week to assess the situation, but say work is ahead of schedule, so the new elevated highway is still expected to open in the summer of 2005.

Yet no amount of science can explain away a traffic nightmare that could last days or weeks on an expressway that carries 75,000 cars a day between Brandon and Tampa.

This morning, traffic will narrow to one lane in each direction past the damaged section of the expressway between 50th and 39th streets. At 50th Street, traffic will detour from the expressway - only to re-enter a few feet away at the east or west-bound ramps - to clear a path for construction equipment. Commuters will not pay tolls at the 50th Street booths.

The single-lane traffic flow aims to relieve congestion along alternative routes from the south Hillsborough suburbs to Tampa, including State Road 60 and Causeway Boulevard.

Expressway authorities sought to assure skittish commuters that the new road will be safe. They said extensive soil tests were conducted prior to construction at more than 200 planned locations for the support columns. At this particular pier site, surveys conducted 73 feet under ground level - about 10 feet below the pier's reach - did not detect concerns.

"The bottom line is, if you have a sinkhole as deep as this one, it is essentially undetectable," said Expressway Authority Executive Director Pat McCue.

He said that experts believed the sinkhole opened under the weight of a piece of 350,000-pound equipment, called a truss, used to connect segments of the elevated expressway. Bumper-to-bumper traffic would amount to a fraction of the load, McCue said.

The Expressway Authority declined Tuesday to release the soil reports conducted by a private company hired by URS Corp., formerly known as URS Greiner. URS worked extensively on Tampa International Airport and many state and local projects.

Experts said soil tests can miss sinkholes, formally defined as a circular depression in an area of karst, or limestone, that can dissolve in groundwater. Tests indicated the pier that sank was anchored in a bed of limestone about 40 feet below ground level.

"A relatively small problem with the soil, something from five to 20 feet in size, is easy to miss, and if you tried to find every one of them, you couldn't afford the project," said Ted Smith, director of geologic hazards at BCI Engineers in Lakeland, a sinkhole specialist. "Sometimes the preferable alternative after you've done the required testing is to build the structure and correct whatever you encounter along the way."

Yet the sinkhole that brought down a football-field-sized segment of the elevated roadway marked the second such hole to surface during the three-year expressway project.

Hubbard Construction Co., which is rebuilding the ground-level lanes of the Crosstown, encountered a sinkhole last October, which delayed that part of the project. Experts disagreed over whether that might have been a warning of more trouble ahead.

The latest problem comes as the project's builder, PCL, faces increasing state scrutiny for a series of structural blunders in the Clearwater bridge and another project in Sarasota.

"You've got to begin to question the work being done by the contractor," said Scott, a member of the Expressway Authority. Until Tuesday, he was unaware the Crosstown project used the same contractor at the center of concerns in Clearwater.

Built like a giant puzzle, the Crosstown project will string together 3,000 pieces precast at the Port of Tampa and then fit together like Legos along the existing Crosstown's six-mile median.

When complete, the elevated road will have three lanes going from Brandon to Tampa in the morning. During evening rush hour, the lanes will reverse to bring SunPass commuters back home from downtown - a project that marks PCL's biggest undertaking in Florida.

For Brandon area commuters, the lack of alternatives became abundantly clear on Tuesday, as thousands of drivers idled in hours-long traffic jams.

Normally, Jon Owens can make the drive from his home in Valrico to MacDill Air Force Base in South Tampa in 45 minutes. On Tuesday, he idled for an hour before reaching a detour at 50th Street, where the expressway closed to traffic for several hours.

"I think it will be okay once they get all the kinks worked out," he said. "Valrico's growing too fast. They need more roads."

In traffic, he was stuck with McCue, who lives in Apollo Beach. The Expressway Authority executive director promised the elevated roadway would open as scheduled next summer.

He had no doubts about the safety of the completed roadway - but could not predict if Mother Nature would trigger another ugly distraction during the construction phase.

"There are sinkholes throughout Florida, McCue said. "There is no such thing as an assurance that it won't happen again."

- Times staff writers Jennifer Farrell, David Karp and Bill Varian and researchers Kitty Bennett and John Martin contributed to this report.

RECORDS REQUEST DENIED

The Tampa-Hillsborough Expressway Authority, a public agency whose board members include city and county elected officials, kept several public documents private Tuesday.

Under Florida's public records law, the St. Petersburg Times submitted a written request for records related to the ground testing for the elevated portion of the Lee Roy Selmon Crosstown Expressway. Those public records would include the name of the subcontractor that performed the tests, details about the tests and who approved them.

The expressway authority refused to release those records and indicated they will be available today.

Q&A

What will traffic on the Crosstown Expressway be like today?

Better than Tuesday, but still a mess. Leave early and consider alternative routes. Two lanes of the Crosstown will be open, but there is a detour at the 50th Street exit that will route drivers off the road and back on in a looping detour. Commuters will not have to pay tolls there.

How long will it take to repair the damage?

It is unclear how long the work will take to complete. But it is expected to take a week to assess the situation and decide how to proceed.

Will the project still open on time in 2005?

Yes, the expressway authority says, because construction was running ahead of schedule.

Will the elevated roadway be safe when completed?

The expressway authority says the road will be safe because even bumper-to-bumper traffic will not weigh as much as a 350,000-pound piece of equipment that caused a sinkhole to open under a piling.

Why wasn't the ground tested for sinkholes?

Soil tests were conducted before construction and did not detect any problems. But experts say problems this deep and relatively small are sometimes tough to spot.

Who is the road builder?

PCL Civil Constructors, which also is building the problem-plagued Memorial Causeway Bridge in Clearwater.

What is its responsibility in Tuesday's collapse on the Crosstown project?

Too early to tell. Another firm conducted the soil tests.

Have previous issues arisen during the Crosstown construction?

Yes. Inspectors discovered PCL did not build columns to specifications, but a review found it installed more steel reinforcements than were called for in the bridge plans. Hubbard Construction Co., which is rebuilding the ground-level lanes of the Crosstown, encountered a sinkhole in October, which delayed that part of the project.